Source-Responsive Modeling of Preferential and Matrix Flow During Infiltration.

Two major influences on water and solute transport in soil are (1) the partitioning of incoming water between preferential and matrix domains, and (2) the distribution of water, especially the preferentially flowing portion, within the profile. For example, a water table may fluctuate rapidly in response to preferential flow, and soil water content may increase nonuniformly depending on preferential flow paths and the degree to which they are active. In fields with drain systems, preferential flow may strongly affect the timing and amount of drain effluent. The source-responsive model, which uses the fact that rapidly-transported flows are particularly sensitive to the temporal and spatial distribution of incoming water, can predict fluxes of preferential flow, and can be combined with a traditional Darcy-Buckingham-Richards model for matrix flow. The characterizations that the source-responsive model requires are (1) the macropore facial area as a function of depth, and (2) the active area fraction, indicating how much of the soil’s preferential flow capacity is conducting flow at a given depth and time. Parameter values can be estimated using measured water contents during an episode of copious infiltration. Tested with long-term records that include a variety of infiltration and redistribution events, this model reasonably captures features of soil-water dynamics that are frequently underrepresented by most models of soil-water flow. For example the model predicts rapid responses of water-table level and drainage outflow, and increases of water content that occur at deeper depths before intermediate depths. Estimated values of the characterizing properties indicate when and where the effects of infiltration-induced preferential flow are likely to be most pronounced.